Various products, equipment, and instrumentation use wavelength swept laser technology as a source for electromagnetic emission. For example, swept source optical coherence tomography (SS-OCT), alternately called optical frequency domain imaging (OFDI), uses a wavelength swept laser for interferometric imaging and ranging. Another example, infrared laser spectroscopy, uses a wavelength swept laser to perform spectroscopy. Different material systems have been used as the gain medium for wavelength swept lasers. The gain medium material, processing of the gain medium material, operating environment, and pump conditions determine the optical gain characteristics and an associated upper bound on the wavelength range over which the gain medium is effective. The design of the laser and reflectivity of the laser cavity mirrors also limit the range of wavelengths that a swept laser supports. In practice, it is often the case that a single tunable laser cannot achieve the desired wavelength range for a given application. A solution can be found to extend the effective wavelength sweep range by combining the outputs of two or more individual lasers.
A U.S. Pat. No. 7,324,569 B2, “Method and system for spectral stitching of tunable semiconductor sources,” teaches a multi semiconductor source tunable spectroscopy system that has two or more semiconductor sources for generating tunable optical signals that are tunable over different spectral bands. The system enables the combination of these tunable signals to form an output signal that is tunable over a combined band including these individual spectral bands of the separate semiconductor sources.
A U.S. Pat. No. 7,554,668 B2, “Light source for swept source optical coherence tomography based on cascaded distributed feedback lasers with engineered band gaps,” teaches a tunable semiconductor laser for swept source optical coherence tomography, comprising a semiconductor substrate; a waveguide on top of said substrate with multiple sections of different band gap engineered multiple quantum wells (MQWs); a multiple of distributed feedback (DFB) gratings corresponding to each said band gap engineered MWQs, each DFB having a different Bragg grating period; and anti-reflection (AR) coating deposited on at least the laser emission facet of the laser to suppress the resonance of Fabry-Perot cavity modes. Each DFB MQWs section can be activated and tuned to lase across a fraction of the overall bandwidth as is achievable for a single DFB laser and all sections can be sequentially activated and tuned so as to collectively cover a broad bandwidth, or simultaneously activated and tuned to enable a tunable multi-wavelength laser. The laser hence can emit either a single lasing wavelength or a multiple of lasing wavelengths and is very suitable for swept-source OCT applications.
A U.S. Pat. No. 8,665,450 B2, “Integrated dual swept source for OCT medical imaging,” teaches an optical coherence analysis system comprising: a first swept source that generates a first optical signal that is tuned over a first spectral scan band, a second swept source that generates a second optical signal that is tuned over a second spectral scan band, a combiner for combining the first optical signal and the second optical signal to form a combined optical signal, an interferometer for dividing the combined optical signal between a reference arm leading to a reference reflector and a sample arm leading to a sample, and a detector system for detecting an interference signal generated from the combined optical signal from the reference arm and from the sample arm.
A U.S. Pat. No. 8,687,666 B2, “Integrated dual swept source for OCT medical imaging,” teaches an optical coherence analysis system comprising: a first swept source that generates a first optical signal that is tuned over a first spectral scan band, a second swept source that generates a second optical signal that is tuned over a second spectral scan band, a combiner for combining the first optical signal and the second optical signal for form a combined optical signal, an interferometer for dividing the combined optical signal between a reference arm leading to a reference reflector and a sample arm leading to a sample, and a detector system for detecting an interference signal generated from the combined optical signal from the reference arm and from the sample arm. In embodiments, the swept sources are tunable lasers that have shared laser cavities.
A U.S. Pat. No. 8,908,189 B2, “Systems and methods for swept-source optical coherence tomography,” teaches systems and methods for increasing the duty cycle and/or producing interleaved pulses of alternating polarization states in swept-source optical coherence tomography (OCT) systems. Embodiments including improved buffering, frequency selecting filter sharing among multiple SOAs, intracavity switching, and multiple wavelength bands are described. The unique polarization properties of the source configurations have advantages in speckle reduction, polarization-sensitive measurements, polarization state dependent phase shifts, spatial shifts, and temporal shifts in OCT measurements.
A U.S. Pat. No. 8,873,066 B2, “System and method for improved resolution, higher scan speeds and reduced processing time in scans involving swept-wavelength interferometry,” teaches a system and method for measuring an interferometric signal from a swept-wavelength interferometer by scanning a tunable laser source over two wavelength ranges, whose centers are separated substantially more than the length of wavelength ranges. The spatial resolution of the measurement is determined by the inverse of the wavelength separation between a first and second wavelength region, as well as by the wavelength range of the first and second regions. An electronically tunable laser may be utilized to produce two wavelength ranges that are widely separated in wavelength. Such a system and method has wide applications to the fields of optical frequency domain reflectometry (OFDR) and swept-wavelength optical coherence tomography (OCT), for example.
A US Patent Application, US 20140268050, “Tunable laser array system,” teaches a system for swept source optical coherence tomography, the system including a light source emitting multiplexed wavelength-swept radiation over a total wavelength range, the light source including N wavelength-swept vertical cavity lasers (VCL) emitting N tunable VCL outputs having N wavelength trajectories, a combiner for combining the N tunable VCL optical outputs into a common optical path to create the multiplexed wavelength-swept radiation, a splitter for splitting the multiplexed wavelength-swept radiation to a sample and a reference path, an optical detector for detecting an interference signal created by an optical interference between a reflection from the sample and light traversing the reference path, and a signal processing system which uses the interference signal to construct an image of the sample, wherein at least one of the N wavelength trajectories differs from another of the N wavelength trajectories with respect to at least one parameter.
Various apparatus and methods using a single wavelength swept laser are described next for purposes of review.
A U.S. Pat. No. 8,705,047 B2, “Optical coherence tomography imaging system and method,” teaches an optical imaging system that includes an optical radiation source, a frequency clock module outputting frequency clock signals, an optical interferometer, a data acquisition (DAQ) device triggered by the frequency clock signals, and a computer to perform multi-dimensional optical imaging of the samples. The frequency clock signals are processed by software or hardware to produce a record containing frequency-time relationship of the optical radiation source to externally clock the sampling process of the DAQ device.
A paper, “Doppler velocity detection limitations in spectrometer based versus swept-source optical coherence tomography” by H. C. Hendargo, R. P. McNabb, A. Dhalla, N. Shepherd, and J. A. Izatt, Biomedical Optics Express, Vol. 2, No. 8, published Jul. 6, 2011, teaches a swept source OCT system in which phase stabilization was performed in real time with the use of an external wavelength reference. The paper teaches that in order to compensate for the phase errors induced by fluctuations in the data acquisition trigger generated by the light source, an external fiber Bragg grating with a narrow linewidth (OE Land, λo=989 nm, Δλ=0.042 nm) was used to trigger the start of the acquisition for each wavelength sweep. The experimental apparatus for phase stabilization is shown in FIG. 2A of the paper.
A paper, “Phase-stabilized optical frequency domain imaging at 1-μm for the measurement of blood flow in the human choroid” by B. Braaf, K. A. Vermeer, V. A. D. P. Sicam, E. van Zeeburg, J. C. van Meurs, and J. F. de Boer, Optics Express, published Oct. 5, 2011, teaches that in optical frequency domain imaging (OFDI) the measurement of interference fringes is not exactly reproducible due to small instabilities in the swept-source laser, the interferometer and the data-acquisition hardware. The resulting variation in wavenumber sampling makes phase-resolved detection and the removal of fixed-pattern noise challenging in OFDI. The paper teaches a post-processing method in which interference fringes are resampled to the exact same wavenumber space using a simultaneously recorded calibration signal. This method is implemented in a high-speed (100 kHz) high-resolution (6.5 μm) OFDI system at 1-μm and is used for the removal of fixed-pattern noise artifacts and for phase-resolved blood flow measurements in the human choroid.
A paper, “Efficient sweep buffering in swept source optical coherence tomography using a fast optical switch” by A. Dhalla, K. Shia, and J. Izatt, Biomedical Optics Express, Vol. 3, No. 12, published Oct. 31, 2012, further teaches the fiber Bragg grating phase stabilization approach of the previously mentioned 2011 H. C. Hendargo paper and teaches a buffering technique for increasing the A-scan rate of swept source optical coherence tomography (SSOCT) systems. Numerical compensation technique are used to modify the signal from a Mach-Zehnder interferometer (MZI) clock obtained from the original sweep to recalibrate the buffered sweep, thereby reducing the complexity of systems employing lasers with integrated MZI clocks.
A US Patent Application, US 20140028997, “Agile Imaging System,” teaches an agile optical imaging system for optical coherence tomography imaging using a tunable source comprising a wavelength tunable VCL laser. The tunable source has long coherence length and is capable of high sweep repetition rate, as well as changing the sweep trajectory, sweep speed, sweep repetition rate, sweep linearity, and emission wavelength range on the fly to support multiple modes of OCT imaging. The imaging system also offers new enhanced dynamic range imaging capability for accommodating bright reflections. Multiscale imaging capability allows measurement over orders of magnitude dimensional scales. The imaging system and methods for generating the waveforms to drive the tunable laser in flexible and agile modes of operation are also described.
A U.S. Pat. No. 8,836,953 B2, “OCT system with phase sensitive interference signal sampling,” teaches an OCT system and particularly its clock system that generates a k-clock signal but also generates an optical frequency reference sweep signal that, for example, indicates the start of the sweep or an absolute frequency reference associated with the sweep at least for the purposes of sampling of the interference signal and/or processing of that interference signal into the OCT images. This optical frequency reference sweep signal is generated at exactly the same frequency of the swept optical signal from sweep to sweep of that signal. This ensures that the sampling of the interference signal occurs at the same frequencies, sweep to sweep. Such a system is relevant to a number of applications in which it is important that successive sweeps of the swept optical signal be very stable with respect to each other.
While many applications potentially benefit from using the output from multiple wavelength swept lasers and there is teaching on generating light from multiple wavelength swept lasers, there is little to no discussion or experimental demonstration of how to effectively detect and process the light from multiple wavelength swept laser sources. In addition to generating the light from multiple wavelength swept lasers, there is a need to effectively detect and combine the information produced by the multiple wavelength swept lasers into a useful signal that properly merges the signals and data. The detection and merging of signals and data is a nontrivial component of a practical apparatus that utilizes multiple wavelength swept lasers, especially under conditions where the lasers may operate at different or varying speeds, at different or varying repetition rates, over different or varying sweep ranges, and over different or varying sweep trajectories, or when the swept laser exhibits sweep-to-sweep variation in the wavelength sweep trajectory. The essential apparatus and methods taught in the present application solve these deficiencies of systems with multiple VCL sources or other wavelength swept sources and also teach the alignment of the interferometric phase, the wavelength, or wavenumber of multiple sequential sweeps of a single VCL source or other single wavelength swept source. The apparatus and methods of the present invention have benefits in reduced computation, higher robustness to noise, and greater flexibility in operating mode of the VCL source or other wavelength swept source.